. >> reporter: a third former president-- george w. bush-- was unable to attend today, as he recovers from a recent heart procedure. instead, he issued a statement saying: the moment that dr. king delivered his famous address-- with the appeal to "let freedom ring"-- was marked by a bell- ringing ceremony. that set the stage for the first african american president, who said the struggle for economic opportunity remains the nation's great unfinished business, but he voiced hope. there's a reason why so many who marched that day, and in the days to come, were young-- for the young are unconstrained by habits of fear, by the conventions of what is. they dared to dream differently, to imagine something better. and i am convinced that same imagination, the same hunger of purpose, stirs in this generation. we may not face the same dangers of 1963, but the fierce urgency of now remains. we may never duplicate the swelling crowds and dazzling procession of that day so long ago-- no one can match king's brilliance but the same flame can light